Perhaps this phrase from the UK fourth F-35B report needs to be fleshed out:
"...This fourth jet, which is specially designed to be a test aircraft..."
I have seen comments that if the 'test' aircraft are required back in the operational fleet then, as is the situation for any LRIP aircraft being upgraded airframe and otherwise, there is sense in going through that process at the appropriate time; which may minimise expense of same. Probably I would have thought the 'test' aircraft are going to be useful for UK specific future testing - even when the overall F-35 tests are over? Perhaps they will be useful for UK specific tests back in UK such as SRVLs and whatnots.